1. Field of the Invention
The invention in general relates to navigation systems, and more particularly to an underwater system for determining range and bearing between two points.
2. Description of the Prior Art
A transponder basically is a device which, upon receipt of an interrogating signal, will transmit its own identifiable signal. Acoustic transponders may be utilized in the underwater environment for navigation purposes. For example, a transponder interrogator, or interrogator, at a certain location can transmit an acoustic interrogating pulse signal to the transponder at a distant location. In one common type of interrogator apparatus a counter is started at the time of transmission. The distant transponder receives the interrogating signal and in response thereto transmits its own identifiable signal which is received by the interrogator and utilized to turn off the running counter. With a known speed of sound in the water, the count will provide an indication of the distance between interrogator and transponder.
The interrogator may be carried by a surface or subsurface vessel with the transponder position at known coordinates on a map. The arrangement however only provides range information and not bearing information. Bearing information may be obtained to compute a precise location by providing a plurality of transponders at known locations each operable to provide an individually identifiable signal distinct from the other transponders and each triggerable upon receipt of the same interrogating signal from the interrogator. Knowing the range to each transponder and their precise relative positions, the precise position of the interrogator may be determined.
Another method of obtaining bearing information without the requirement for using a plurality of transponders is to provide an interrogator which rotates and is indexed through 360.degree. and transmits an interrogating signal with an extremely narrow beam pattern during rotation. Such apparatus generally is bulky, consumes power, and requires rotating seals.
The apparatus of the present invention functions to determine range and bearing utilizing only a single transponder and without the requirement for rotation of the interrogating portion of the system. The apparatus is lightweight, compact and the interrogator portion can be carried not only by surface and subsurface vessels but can also be utilized by free swimmers or small sports submersibles for homing in on a specific location such as a base station.